1. Field of the Invention
The present invention relates to a dielectric filter and a dielectric duplexer for use in a communication base station or the like.
2. Description of the Related Art
A dielectric filter relating to the present invention is shown in FIG. 7. Although this dielectric filter is described in copending U.S. patent application Ser. No. 924,040, now U.S. Pat. No. 6,052,041, this dielectric filter was not known yet in the art at the time when the invention, on the basis of which the priority of the present invention is claimed, was filed as Japanese Patent Application No. H-10-8860.
FIG. 7 is a perspective view of the dielectric filter 110. In this figure, an upper lid 114 is removed so that the internal structure can be seen. The dielectric filter shown herein is, by way of example, of the two-stage band-rejection filter type including two disk-shaped dielectrics 112 disposed side by side in a shielding cavity frame 111.
The dielectric filter 110 includes a shielding cavity frame 111 formed of metal, a dielectric 112 disposed in the shielding cavity frame 111, and external coupling means 120. The dielectric 112 is formed of ceramic in a disk shape and electrodes are formed of silver or the like on two opposite upper and lower surfaces thereof. The lower surface of the dielectric 112 is fixed via solder or the like to the inner bottom surface of the shielding cavity frame 111 thereby achieving electric connection. The external coupling means 120 includes an electric probe 121 made of a metal wire. The electric probe 121 is disposed in such a manner that it extends in a space between the upper surface of the dielectric 112 and the shielding cavity frame 111 without having contact with either the dielectric 112 or the shielding cavity frame 111. By employing the above structure, it becomes possible to reduce the current flowing through the shielding cavity frame 111 thereby reducing the loss due to such a current thus achieving a TM-mode dielectric filter having a small size in height and having high unloaded Q.
The electric probe 121 is connected via solder to an external connector 113 attached to the shielding cavity frame 111 so that a signal is input and output via the electric probe 121. That is, a signal is supplied via a cable connected to the external connector 113 and is passed through the electric probe 121. The electric probe 121 is coupled with the dielectric 112 via capacitance created between the electric probe 121 and the electrode of the dielectric 112. The dielectric 112 coupled with the electric probe 121 has resonance and thus serves as a band-rejection filter. The resultant signal is output through a cable connected to another external connector 113.
In the above-described dielectric filter, the coupling between the electric probe and the dielectric is realized via the capacitance between the electric probe and the dielectric. Thus, the strength of the coupling depends on the capacitance between the electric probe and the dielectric. The capacitance is determined by the distance between the electric probe and the dielectric, the areas of surfaces facing each other, and the dielectric constant of a substance existing between them. The attenuation varies with the change in the strength of the coupling, and the characteristic of the dielectric filter vary in such a manner that the filter has a band-rejection or bandpass characteristic with a wide bandwidth when the coupling is strong while the bandwidth becomes narrow when the coupling is weak. Therefore, to achieve a dielectric filter having desired characteristics, it is required to adjust the coupling between the electric probe and the dielectric. That is, it is required to adjust the capacitance between the electric probe and the dielectric.
In the above-described dielectric filter, the electric probe made of a metal wire is used as the external coupling means. The capacitance may be varied, as described above, by varying the distance between the electric probe and the dielectric, the areas of the surfaces, and/or the dielectric constant. However, it is difficult to change the location of the electric probe connected to the external connector because it is difficult to change the location of the external connector attached to the shielding cavity frame. The dielectric constant of air is impossible to change. Thus, a most practical manner of adjusting the capacitance between the electric probe and the dielectric is to change the length of the electric probe thereby changing the areas of the surfaces facing each other.
However, it is a troublesome process to adjust the length of the electric probe by cutting the electric probe for each dielectric or for each dielectric filter including the dielectric. Furthermore, if once the electric probe is cut to a too short length, it is impossible to make a readjustment to increase the capacitance.
Furthermore, an electrical discharge occurs through air existing between the electric probe and the dielectric when the potential difference between them exceeds the dielectric strength of air. Such a discharge can cause a difference in the characteristic of the electrode or the electric probe of the dielectric filter.
In view of the problems described above, it is an object of the present invention to provide a dielectric filter and dielectric duplexer whose characteristics can be more easily adjusted and which have higher reliability.